A natural hydrogen carbonate play: Sandino Forearc Basin, Pacific offshore Nicaragua

The Sandino forearc basin of Pacific offshore Nicaragua forms part of the Central American convergent margin where the Cocos Plate is subducting beneath the crust of Central America. In subduction zones, aqueous fluids are released by dehydration processes in the subducting slab and sediments. These fluids have the potential to cause serpentinization and generate hydrogen in ultramafic rocks of both the crust and mantle above the subduction zone (the ‘upper plate’). Geophysical data are used to investigate the hydrogen system formed in this forearc setting. Gravity-seismic modeling along the margin has shown that dense ophiolitic rocks form the crust of the upper plate, and mantle rocks of the mantle wedge come to within 10 km of the bottom of the forearc basin. Both rock types constitute potential protoliths for serpentinization by aqueous fluids driven off from the hydrous downgoing Cocos slab. The configuration and thermal structure of the convergent margin offshore Nicaragua are such that serpentinization of ultramafic protoliths and hydrogen generation is predicted to be occurring below the basin and outer high of the forearc. The hydrogen generated will move upward by buoyancy and access the sediments of the basin via faults and fracture networks formed by accretionary tectonics. Late Cretaceous to early Tertiary shallow-water carbonates are known from outcrop and are interpreted from 2D and 3D seismic data in the forearc basin. These carbonates have reservoir potential and form part of the basal section of the basin along the flanks of the outer high. Thus, they are ideally placed to be charged by hydrogen emanating from the basement. The shallow-water carbonates form a natural hydrogen exploration play in the Sandino Basin with a fairway definable from seismic interpretation.